This has been said before, but if you're looking for a western/scientific explanation for the internal stuff---as someone who's been practicing it for a bit, I find the idea of fascial contraction and tensegrity to be very compelling. I don't think it explains it 100%, but it goes a LONG way towards explaining how "relaxed" and "unmuscle-y" movement might be possible. (But again, knowing the physiology behind it doesn't really do anything towards explaining how a student does it on a practical level.)